Mach Number

Mach number compares how fast an aircraft is moving through the air with how fast pressure disturbances can travel in that same air. Because the speed of sound depends on the local temperature and gas properties, Mach number is a local, dimensionless measure that tells you whether the flow is subsonic, transonic, supersonic, or hypersonic. It is useful because many aerodynamic effects, such as compressibility and shock waves, depend more on this ratio than on speed alone.

Use this formula whenever you need to determine an aircraft’s speed regime or compare its speed to the local sonic speed. It is especially important in high-speed flight, wind-tunnel testing, and performance calculations where temperature changes alter the speed of sound.

• Using the speed of sound at sea level instead of the local speed of sound
• Mixing units, such as mph for v and m/s for a
• Assuming Mach number is always a fixed speed rather than a ratio that changes with atmospheric conditions